We shall undertake studies on the role of siroheme and iron-sulfur centers in the process of sulfite reduction using rapid kinetic techniques associated with electron paramagnetic resonance and optical spectroscopy. These studies will be extended to an examination of the nature and functional role of iron-sulfur centers in the mitochondrial respiratory chain of yeast and its mutants. We shall undertake to determine the complete chemical structure of siroheme using high resolution NMR, mass spectroscopy, and X-ray diffraction techniques. We shall compare that structure to that of a similar tetrapyrrole which may serve as an important intermediate in the pathway that leads from porphyrins to corrins (B12 derivatives). BIBLIOGRAPHIC REFERENCES: J.M. Vega, R.H. Garrett, and L.M. Siegel (1975) J. Biol. Chem. 250: 7880-7989. Siroheme: A prosthetic group of Neurospora crassa assimilatory nitrite reductase. D.C. Rueger and L.M. Siegel (1976) in "Flavins and Flavoproteins" (T.P. Singer, ed.), pp. 610-620. Elsevier, Amsterdam. Escherichia coli sulfite reductase: Role of siroheme sulfur complexes in the multielectron reduction process.